In research magnets or MRI magnets, there is demand for adjustment of a magnetic field distribution with high accuracy such that a constant magnetic field value or a target magnetic field distribution is achieved. For example, in a magnetic resonance imaging magnet (MRI, nmr), a magnetic field distribution which is constant at accuracy of 1 ppm or less and which is constant over time may be required.
When a superconductive coil is employed as a main magnetomotive force source and is supplied with a current in a permanent current mode, the magnetic flux therein is maintained and the magnetic field strength is maintained constant. A superconductive coil is used for a magnet of an MRI device or the like using such characteristics and a magnetic field is maintained substantially constant for a long time of one year or more. The MRI device also includes a fine-adjustment coil that can finely adjust a distribution of a magnetic field in a center axial direction of the magnet.
FIG. 12 illustrates a conventional fine-adjustment coil (a local coil) disclosed in PTL 1. A plurality of local coils are arranged at intervals. Accordingly, fine adjustment of a magnetic field is possible. A current distribution of each coil is calculated using a strength of a spherical surface harmonic function and a Lagrangian method of undetermined coefficients.
FIG. 13 illustrates wiring of a fine-adjustment coil disclosed in PTL 2. By allowing a current to flow in wires disposed on a cylindrical surface, a magnetic field is adjusted. It is noted that current values are determined using singular value decomposition.